Variable-polarity plasma arc welding process and device

ABSTRACT

The invention relates to a plasma arc welding process and a plasma arc welding device for aluminum and its alloys, which process comprises at least one step of initiation of a pilot arc by means of an auxiliary current which is delivered by an auxiliary current source and flows in a pilot-arc circuit, one step of feeding with plasma gas, one step of initiation of a main arc by means of a main current which is delivered by a main current source and flows in a main circuit, one step of cyclic variation of the polarity of the main current so as to obtain at least one phase of descaling the material to be welded with a first current polarity and at least one phase of welding the material with a second current polarity, and in which process the supply of auxiliary current to the pilot-arc circuit is stopped during the descaling phase and the main circuit and to the pilot-arc circuit are supplied with main current.

FIELD OF THE INVENTION

The invention relates to a plasma arc welding process for aluminum andaluminum alloys.

BACKGROUND OF THE INVENTION

Plasma arc welding or plasma welding is, as taught by the document"Plasma Arc Welding", Chap. 10, pages 330-350, 8th edition 1991,incorporated here by way of reference, a metal fusion welding processwhich involves heating the metal by means of an electric arc which formsbetween a non-consumable electrode and the metal workpiece or workpiecesto be welded (transferred arc) or, depending on the case, between theelectrode and the nozzle of the plasma torch (non-transferred arc).

An ionized gas flow delivered by the plasma torch serves, on the onehand, as a gas shield and, on the other hand, to transfer the heatgenerated by the electric arc to the workpiece to be welded and,optionally, to channel the electric arc between the non-consumableelectrode and the workpiece to be welded.

An inert gas, such as argon or a mixture comprising such a gas, forexample an argon/helium mixture, may be employed as the plasma gas.Usually, plasma arc welding processes are used to weld "carbon" steelsor stainless steels in a single pass, that is to say in a single run,and without any special preparation beforehand of the steel workpiecesto be welded, this being the case for thicknesses ranging up to 10 mm.

Thus, in "keyhole" welding mode, especially by dint of a high energydensity and of constriction of the arc, a hole is formed in line withthe weld joint and the flow of plasma gas penetrates reliably throughthe entire thickness of the material.

Moreover, aluminum and aluminum alloys are conventionally welded byinert-gas-shielded arc welding processes, such as the TIG (TungstenInert Gas) or MIG (Metal Inert Gas) type processes rather than by usingplasma arc type processes.

However, there has for some time now been a strong demand from industry,particularly from the aeronautical and aerospace industries, for plasmaarc welding processes allowing aluminum and aluminum alloys to beeffectively welded, in particular using automatic welding.

Thus, plasma arc welding in "keyhole" mode, which was firstly usedwidely for welding carbon and stainless steels, has been extended to thewelding of aluminum and of aluminum alloys.

The document "Plasma Arc Welding of Aluminum Gas Containers", by H.Fostervoll et al., pages 367-375, incorporated here by way of reference,describes a process for the plasma arc welding in "keyhole" mode of twoaluminum hemispheres or half-containers so as, after joining and weldingthem together, to form a gas container 352 mm in diameter, the aluminumwalls of which have a thickness of 8 mm. This process uses avariable-polarity welding set of the HOBART™ VP-300-S type, a weldingcontrol system of the ISOTEK™ type and 99.99% pure argon as the plasmagas.

Furthermore, the document "Variable Polarity Plasma Arc Welding on theSpace Shuttle External Tank", by A. C. Numes et al., Welding Journal,September 1984, pages 27-35, describes a variable-polarity plasma arcwelding process used for NASA in order to produce the external tanks ofthe American space shuttle. This document particularly stresses the lowcost of the plasma arc process, compared with conventional TIG or MIGprocesses, when welding aluminum, given that this process especiallysaves having to pretreat the workpieces to be welded.

This is because, unlike steel, aluminum and its alloys require, beforewelding, a prior preparation or pretreatment in order to removetherefrom the oxides and other contaminants (dust, grease, etc.) thatare likely to cover them. Usually, this pretreatment is carried out bychemical pickling or mechanical descaling, such as brushing, of the saidaluminum workpieces, which correspondingly increases the productioncosts.

However, it has been observed that by varying, over time, the polarityof the current used during the arc plasma welding it was possible toeffect a kind of descaling of the surface of the material by the plasmaflow, prior to the actual welding of the material.

More specifically, variable polarity is a form of alternating currenthaving an asymmetric, rectangular waveform which can be controlled interms of period and in terms of amplitude.

Usually, the current is varied cyclically from a background current (Id)or descaling-phase current, maintained for a descaling time (Td), to apeak current (Iw) or welding-phase current, maintained for a weldingtime (Tw).

In general, a descaling time (Td) of duration less than the welding time(Tw) is chosen, but with a greater amplitude of the current during thedescaling phase than that during the welding phase.

This is because it has been found that the lifetime of the tungstenelectrode depends on the Tw/Td ratio.

Furthermore, the set-back of the electrode in the nozzle of the weldingtorch, the torch height with respect to the workpiece or material to bewelded, and the plasma gas flow rate play a not insignificant role inthe dynamics of the flow.

Thus, in keyhole-mode welding, it is necessary to control, that is tosay limit, the flow rate of the plasma gas in order to prevent anyadventitious cutting of the workpiece. This may especially beaccomplished by choosing an electrode diameter as small as possible soas not to cause too rapid a gas flow.

On the other hand, a large set-back distance of the electrode in thenozzle and/or a higher current signal frequency make it possible toincrease the stiffness of the plasma column and therefore to decreasethe supply of electrical energy.

Variable-polarity welding also has the advantage of resulting in lowporosity in the weld bead.

SUMMARY OF THE INVENTION

The object of the present invention is to improve the variable-polarityplasma arc welding processes for aluminum and its alloys by providing aprocess and a device:

not having the drawbacks of the conventional processes,

making it possible to obtain better welding stability for very thickworkpieces, i.e. for workpieces having a thickness of 8 mm and higher,for example,

allowing such workpieces to be welded effectively, without workpiecepreparation, that is to say by means of a square butt joint, and

being easy to implement on an industrial scale.

The present invention therefore relates to a plasma arc welding processfor aluminum and aluminum alloys, comprising at least:

one step of initiation of a pilot arc between an electrode and a weldingnozzle by means of an auxiliary current which is delivered by at leastone auxiliary current source and flows in a pilot-arc circuit;

one step of feeding the welding nozzle with plasma gas;

one step of initiation of a main arc between the electrode and amaterial to be welded by means of a main current which is delivered byat least one main current source and flows in a main circuit;

one step of cyclic variation of the polarity of the main current so asto obtain at least one phase of descaling the material with a firstcurrent polarity and at least one phase of welding the material with asecond current polarity, and in which process the supply of auxiliarycurrent to the pilot-arc circuit is stopped for at least part of thedescaling phase and the main circuit and the pilot-arc circuit aresupplied with the main current.

Depending on the case, the process of the invention comprises one ormore of the following characteristics:

the supply of the main current to the pilot-arc circuit is maintainedthroughout the descaling phase;

while the main current is being supplied to the pilot-arc circuit, themain current is left to be distributed naturally in the pilot-arccircuit and in the main circuit on the basis of the equivalentimpedances of these circuits;

while the main current is being supplied to the pilot-arc circuit, themain current source and the auxiliary current source operate in a mannerequivalent to a series operation;

while the main current is being supplied to the pilot-arc circuit, themain current source delivers a main current of at least 100 A,preferably of 150 to 300 A;

the duration of the descaling phase is preferably less than the durationof the welding phase.

The invention furthermore relates to a device capable of implementing aprocess according to the invention, and more particularly a plasma arcwelding device comprising:

a plasma torch having a welding nozzle assembly provided with a nozzleand an electrode, preferably a concentric nozzle for annular shielding;

means for feeding the torch with a plasma gas;

at least one programmable, controllable or pilot-controllable,variable-polarity main current source;

a main current circuit;

at least one auxiliary current source;

a pilot-arc circuit;

means for initiating a pilot-arc between the electrode and the nozzle;

means for switching from a pilot arc to a main arc;

control means allowing the polarity of the main current to be variedcyclically between a first current polarity corresponding to adescaling-phase polarity and at least one second current polaritycorresponding to a welding-phase polarity; and

control means allowing the supply of auxiliary current to the pilot-arccircuit to be stopped and allowing the main circuit and the pilot-arccircuit to be supplied with the main current during at least part of thedescaling phase.

The invention also relates to the use of such a device in an operationof welding a workpiece, that is to say a component, element, structureor the like, comprising at least one part made of aluminum or of analuminum alloy, as well as to a component for the chemical,petrochemical or nuclear industry or for rail, sea, air, road or spacetransportation, which includes at least one weld capable of beingobtained by the process of the invention.

The invention will now be described in more detail with the aid of adiagram of one embodiment of a device according to the invention and ofcomparative tests made with such a device, these being given by way ofillustration but being in no way limiting.

BRIEF DESCRIPTION OF THE INVENTION

The appended single FIGURE shows a diagram of a device for implementingthe process of the invention and to check its effectiveness.

More specifically, the plasma arc welding device of the appended singlefigure comprises a plasma torch having a welding nozzle assemblyprovided with a nozzle 1 surrounding a tungsten electrode 2, the nozzle1 itself being inserted into an outer nozzle 3 which provides annularshielding. Feed means 4 feed the torch with plasma gas.

The main current circuit 6 is supplied by a programmablevariable-polarity main current source 5, which is controlled by controlmeans 11, the control means 11 making it possible to vary the polarityof the main current cyclically, between a first current polaritycorresponding to a descaling-phase polarity and at least one secondcurrent polarity corresponding to a welding-phase polarity. The maincurrent source 5 delivers, for example, an alternating current of 320 A.

Likewise, a pilot-arc circuit 8 is supplied by an auxiliary currentsource 7.

Moreover, pilot-arc initiation means 9 make it possible to initiate apilot arc 15 between the electrode 2 and the nozzle 1 and main-arcinitiation means make it possible to initiate a main arc 16 between thesaid electrode 2 and the material 17 to be welded, so as to obtain aweld joint 18.

During all or part of the descaling phase, control means 10 ensure thatthe arc-pilot circuit 8 stops being supplied with auxiliary current andthat the main circuit 6 and the pilot-arc circuit 8 are supplied withthe main current.

In other words, during the descaling phases, the fact of maintaining thearc-pilot circuit 8 connected to the main circuit 6 causes a naturaldistribution of the main current between the two circuits on the basisof their own inductances.

In order to check the effectiveness of the process and of the device ofthe invention, several comparative tests (Tests 1 to 5) were carriedout; the experimental parameters used during these tests are given inthe following table.

                  TABLE                                                           ______________________________________                                        Test No.                                                                              1        2       3       4     5                                      ______________________________________                                        Descaling                                                                             220      250     310     200   180                                      current (A)                                                                   Descaling 3.5 3.5 3.5 3.5 3.5                                                 time (msec)                                                                   Welding 140 180 180 140 140                                                   current (A)                                                                   Welding 8 8 8 8 8                                                             time (msec)                                                                   Arc length 10 to 10 to 10 to 3 3                                              (mm) 12 12 12                                                                 Flow rate 3 3 3 3.5 3                                                         (l/min.)                                                                      Voltage (V) 30 31 31 28 28                                                    Welding 24 22 22 25 25                                                        speed                                                                         (cm/min.)                                                                     Flow rate 20 20 20 20 20                                                      (l/min.)                                                                      Type of NIC 30 NIC 10 NIC 30 NIC 30 NIC 30                                    wire                                                                          Wire 1.2 1.2 1.2 1.6 1.6                                                      diameter                                                                      (mm)                                                                          Pay-out 1.4 1.4 1.4 1.8 1.8                                                   speed                                                                         (m/min.)                                                                      Nozzle 3 3 3 3 3                                                              diameter                                                                      (mm)                                                                          Plate 6 6 8 6 6                                                               thickness                                                                     (mm)                                                                          Alloy type AG4 A5 AG4 AG4 AG4                                                 Pilot arc no no no no no                                                      operating                                                                     during                                                                        welding                                                                       Pilot arc yes yes yes no no                                                   operating                                                                     during                                                                        descaling                                                                   ______________________________________                                    

In Tests 1 to 5, the annular gas used is an argon/helium mixture, suchas the gas INARC 6™, and the plasma gas used is argon, such as the gasARCAL 1™; the INARC 6™ and ARCAL 1™ gases are sold by L'AIR LIQUIDE.Furthermore, the NIC10- and NIC30-type wires are sold by LA SOUDUREAUTOGENE FRANCAISE and correspond to aluminum alloys of the 1100 typeand 5356 type, respectively.

The results obtained are as follows:

Test 1 (downhand or flat-position welding): it is observed that thesurface of the weld bead is covered with a whitish film. However, weldpenetration is uniform and there is no appearance of undesirableporosity after radiographic inspection;

Test 2 (downhand welding): welding an A5-type aluminum alloy requires alarger amount of energy to be supplied than that for an AG4-type alloy.However, the descaled zone is wide;

Test 3 (downhand welding): the welding regime is stable and the resultsare similar to those obtained in Test 1;

Tests 4 and 5 (horizontal-vertical position welding): given the collapseof the pool of molten metal in the horizontal-vertical position, it ispreferable to use large wire diameters (1.6 mm) and a higher pay-outspeed (1.4 to 1.8 m/min.). For the rest, the quality of the weld beadsis substantially equivalent to that of Tests 1 and 3 (the same operatingparameters) and the weld-toe and weld-root reinforcements and the jointsare satisfactory. As regards the degree of porosity, this is alsosubstantially equivalent to that in Tests 1 and 3.

As a result, for a given thickness of material, the process of theinvention makes it possible to maintain a stable and effective plasmajet during descaling, that is to say avoiding any adventitious cuttingof the material to be welded, and furthermore to give weld beads thathave a slightly oxidized appearance and are free of porosity.

The process of the invention may be used especially for welding aluminumand aluminum alloys having a thickness of about 4 to 10 mm, or more.

We claim:
 1. Plasma arc welding process for aluminum and aluminumalloys, which comprises:initiating a pilot arc between an electrode anda welding nozzle by supply of an auxiliary current which is delivered byat least one auxiliary current source and flows in a pilot-arc circuit;feeding the welding nozzle with plasma gas; initiating a main arcbetween the electrode and a material to be welded by supply of a maincurrent which is delivered by at least one main current source and flowsin a main circuit; cyclically varying the polarity of the main currentso as to obtain at least one phase of descaling the material with afirst current polarity and at least one phase of welding the materialwith a second current polarity; and stopping the supply of auxiliarycurrent to the pilot-arc circuit for at least part of the descalingphase, and supplying the main circuit and the pilot-arc circuit with themain current.
 2. Process according to claim 1, wherein the supply of themain current to the pilot-arc circuit is maintained throughout thedescaling phase.
 3. Process according to claim 1, wherein while the maincurrent is being supplied to the pilot-arc circuit, the main current isleft to be distributed naturally in the pilot-arc circuit and in themain circuit on the basis of the equivalent impedances of thesecircuits.
 4. Process according to claim 1, wherein while the maincurrent is being supplied to the pilot-arc circuit, the main currentsource and the auxiliary current source operate in a manner equivalentto a series operation.
 5. Process according to claim 1, wherein whilethe main current is being supplied to the pilot-arc circuit, the maincurrent source delivers a main current of at least 100 A.
 6. Processaccording to claim 5, wherein while the main current is being suppliedto the pilot-arc circuit, the main current source delivers a maincurrent ranging from 150 to 300 A.
 7. Process according to claim 1,wherein the duration of the descaling phase is less than the duration ofthe welding phase.
 8. Plasma arc welding device comprising:a plasmatorch having a welding nozzle provided with an electrode; means forfeeding the torch with a plasma gas; at least one programmablevariable-polarity main current source; a main current circuit; at leastone auxiliary current source; a pilot-arc circuit; means for initiatinga pilot arc between the electrode and the nozzle; means for switchingfrom a pilot arc to a main arc; control means allowing the polarity ofthe main current to be varied cyclically between a first currentpolarity corresponding to a descaling-phase polarity and at least onesecond current polarity corresponding to a welding-phase polarity; andcontrol means allowing the supply of auxiliary current to the pilot-arccircuit to be stopped and allowing the main circuit and the pilot-arccircuit to be supplied with the main current during at least part of thedescaling phase.